With InDesign closed, try deleting the "current" workspace .xml files from your user library. The workspaces folder should be in Hard Drive/Users/<USER>/Library/Preferences/Adobe InDesign/<Version #>/<Language>/
This is a hidden folder since Lion was released, so if you don't know how to find it, see
Access hidden user library files | Mac OS 10.7 Lion
Thanks for the suggestion Peter, but unfortunately it didn't work. I gave it a shot and the caption bar is still stuck in "long form."
Anything else you can think of that I could try?
Next thing I'd try is deleting the prefs. See Replace Your Preferences
Switching to one of the default workspaces might work, too, but I think it's unlikely. I might also try docking the panel with another, both by stacking it below another one as a "flotilla" and by adding it to a panel group. I'd do this after the prefs, though, as you risk making the other panels wider rather than reducing the width on this one.
Time machine might also be an option.